Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a food grade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means.
Methods for the chemical synthesis of the butanol isomer, isobutanol, are known, such as oxo synthesis, catalytic hydrogenation of carbon monoxide (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719) and Guerbet condensation of methanol with n-propanol (Carlini et al., J. Molec. Catal. A. Chem. 220:215-220, 2004). These processes use starting materials derived from petrochemicals. The production of isobutanol from plant-derived raw materials could minimize the use of fossil fuels and would represent an advance in the art. Microorganisms capable of fermentative production of isobutanol have been described (for example, in U.S. Pat. Nos. 7,851,188 and 7,993,889).
Isobutanol is produced biologically as a by-product of yeast fermentation. It is a component of “fusel oil” that forms as a result of the incomplete metabolism of amino acids by this group of fungi. Isobutanol may be produced from catabolism of L-valine. After the amine group of L-valine is harvested as a nitrogen source, the resulting α-keto acid is decarboxylated and reduced to isobutanol by enzymes of the so-called Ehrlich pathway (Dickinson et al., J. Biol. Chem. 273:25752-25756, 1998). Microorganisms expressing engineered biosynthetic pathways for producing butanol isomers, including isobutanol, are also described (see U.S. Pat. Nos. 7,851,188 and 7,993,889, which are incorporated herein by reference).